Christian calendar calculators

Christian liturgical calendar math: Western and Orthodox Easter plus Ash Wednesday, Good Friday, and Pentecost for any year; first Advent Sunday; live countdown to the next Western Easter with “Easter is today” on the day. Standard computus — not diocesan tables.

Easter is a moveable feast, and the method for finding it — the computus — is why these dates are worth calculating rather than looking up. The rule places Easter on the first Sunday after the ecclesiastical full moon on or after 21 March, using a tabular approximation of the lunar cycle rather than the actual astronomical moon, so the two occasionally differ by a day. Western churches apply that rule on the Gregorian calendar while most Orthodox churches use the Julian calendar, which currently runs thirteen days behind; the two Easters therefore usually fall on different Sundays and occasionally coincide. Once Easter is fixed, the rest of the season follows from it: Ash Wednesday is 46 days before, Good Friday the preceding Friday, and Pentecost 49 days after. Advent is counted back from Christmas instead. Local dioceses may transfer some observances.